Low power radio frequency communication

ABSTRACT

A method, system and tag for low power radio frequency communication are described. In one embodiment, the RF tag comprises: a radio, an energy harvesting unit operable to convert incident RF energy to direct current (DC), a storage unit to store recovered DC power, one or more sensors for sensing and logging data, and a microcontroller coupled to the energy harvesting and storage units, the one or more sensors and the radio, the microcontroller operable to wake up from a sleep state and cause the radio to communicate, sensed data from at least one of the one or more sensors while powered by energy previously harvested and stored by the energy harvesting and storage unit.

PRIORITY

The present patent application claims priority to and incorporates byreference the corresponding provisional patent application Ser. No.61/620,878, titled, “Low Power Radio Frequency Communication,” filed onApr. 5, 2012.

FIELD OF THE INVENTION

Embodiments of the present invention relate to the field ofradio-frequency (RF) communication; more particularly, embodiments ofthe present invention relate to RF tags that perform energy harvesting,collect and process sensor data and communicate data using standard RFcommunication techniques.

BACKGROUND OF THE INVENTION

Radio Frequency Identification (RFID) tags are becoming increasinglycommon. RFID tags that include sensing capabilities have emerged as agenerally inexpensive and effective means of addressing many wirelesssensor applications in both indoor and outdoor sensing applications.Purely passive sensors, such as RFID tags, when actively interrogated byan RF transceiver/reader, receive energy to power themselves up so thatthey can acquire readings from their attached sensing elements.Generally, RFID tags equipped with one or more sensors require a sourceof energy to measure and store their acquired information at times otherthan during active interrogation by a reader. Standard passive(battery-less) RFID tags provide no means of acquiring sensorinformation unless they are being actively interrogated by a reader.

Next generation sensor networks may be powered by energy harvestingtechniques to avoid requiring battery maintenance. Energy harvesting isa process by which energy is derived from external sources (e.g., radiofrequency energy, solar power, thermal energy, wind energy, salinitygradients, or kinetic energy), captured and stored.

Energy may be harvested from radio frequency signals propagatingwirelessly. With RF harvesting, wireless energy conies from a radiofrequency transmitting device that is some distance away from a devicethat harvests energy from the radio frequency transmission.

One of the more popular forms of RF used today is Wi-Fi (also referredto as IEEE 802.11a/b/g/n etc.) communications. Today, most Wi-Ficommunications are in the 2.4 GHz and 5.8 GHz frequency bands and thereare many local area networks that are based on Wi-Fi in which accesspoints enable Wi-Fi clients to gain access to networks such as theInternet. Furthermore, the 2.4 GHz and 5.8 GHz bands also support othernetworking standards, such as Zigbee and Bluetooth, and otherproprietary networks, each transmitting energy by communicating in thissame frequency band. Additionally there are other frequency bands thatsupport different communication protocols, each of which transmit energywhen they are communicating.

SUMMARY OF THE INVENTION

A method, system and tag for low power radio frequency communication isdescribed. In one embodiment, the RF tag comprises: a radio, an energyharvesting unit operable to convert incident RF energy to direct current(DC), a storage unit to store recovered DC power, one or more sensorsfor sensing and logging data, and a microcontroller coupled to theenergy harvesting and storage units, the one or more sensors and theradio, the microcontroller operable to wake up from a sleep state andcause the radio to communicate to a network the sensed data from atleast one of the one or more sensors while powered by energy previouslyharvested and stored by the energy harvesting and storage unit.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detaileddescription given below and from the accompanying drawings of variousembodiments of the invention, which, however, should not be taken tolimit the invention to the specific embodiments, but are for explanationand understanding only.

FIG. 1 illustrates one embodiment of a sensor tag that communicates overWi-Fi.

FIG. 2 is a tag state diagram for one embodiment of the Wi-Ficommunication tag.

FIG. 3 illustrates one embodiment of a tag that communicates usingbackscatter.

FIG. 4 is one embodiment of a backscatter tag state diagram.

FIGS. 5A-5C illustrate a backscatter tag performing backscattercommunication with an access point (AP) in a sensor network.

FIG. 6 illustrates an example of a backscatter-UDP packet.

FIGS. 7-9 illustrate a number of embodiments of access pointenhancements that allow an access point to communicate with abackscatter tag.

FIG. 10 illustrates a method of a nearly-zero-power RF passivepattern-detector wake-up that is frequency-domain specific (nottime-domain specific).

FIG. 11 illustrates a wireless communication system that harvests RFenergy in the 2.4 GHz ISM band, while backscatter communications occurin the 5.8 GHz ISM band.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Embodiments of the present invention include RF tags and wirelesscommunication systems (e.g., Wi-Fi communication systems) that includesuch tags. Such tags may be part of a sensor network. In one embodiment,the tags perform energy harvesting, collect and process sensor data, andcommunicate data using standard RF communication techniques (e.g.,Wi-Fi).

In the following description, numerous details are set forth to providea more thorough explanation of the present invention. It will beapparent, however, to one skilled in the art, that the present inventionmay be practiced without these specific details. In other instances,well-known structures and devices are shown in block diagram form,rather than in detail, in order to avoid obscuring the presentinvention.

Some portions of the detailed descriptions which follow are presented interms of algorithms and symbolic representations of operations on databits within a computer memory. These algorithmic descriptions andrepresentations are the means used by those skilled in the dataprocessing arts to most effectively convey the substance of their workto others skilled in the art. An algorithm is here, and generally,conceived to be a self-consistent sequence of steps leading to a desiredresult. The steps are those requiring physical manipulations of physicalquantities. Usually, though not necessarily, these quantities take theform of electrical or magnetic signals capable of being stored,transferred, combined, compared, and otherwise manipulated. It hasproven convenient at times, principally for reasons of common usage, torefer to these signals as bits, values, elements, symbols, characters,terms, numbers, or the like.

It should be borne in mind, however, that all of these and similar termsare to be associated with the appropriate physical quantities and aremerely convenient labels applied to these quantities. Unlessspecifically stated otherwise as apparent from the following discussion,it is appreciated that throughout the description, discussions utilizingterms such as “processing” or “computing” or “calculating” or“determining” or “displaying” or the like, refer to the action andprocesses of a computer system, or similar electronic computing device,that manipulates and transforms data represented as physical(electronic) quantities within the computer system's registers andmemories into other data similarly represented as physical quantitieswithin the computer system memories or registers or other suchinformation storage, transmission or display devices.

The present invention also relates to an apparatus for performing theoperations herein. This apparatus may be specially constructed for therequired purposes, or it may comprise a general purpose computerselectively activated or reconfigured by a computer program stored inthe computer. Such a computer program may be stored in a computerreadable storage medium, such as, but is not limited to, any type ofdisk including floppy disks, optical disks, CD-ROMs, andmagnetic-optical disks, read-only memories (ROMs), random accessmemories (RAMs), EPROMs, EEPROMs, magnetic or optical cards, or any typeof media suitable for storing electronic instructions, and each coupledto a computer system bus.

The algorithms and displays presented herein are not inherently relatedto any particular computer or other apparatus. Various general purposesystems may be used with programs in accordance with the teachingsherein, or it may prove convenient to construct more specializedapparatus to perform the required method steps. The required structurefor a variety of these systems will appear from the description below.In addition, the present invention is not described with reference toany particular programming language. It will be appreciated that avariety of programming languages may be used to implement the teachingsof the invention as described herein.

A machine-readable medium includes any mechanism for storing ortransmitting information in a form readable by a machine (e.g., acomputer). For example, a machine-readable medium includes read onlymemory (“ROM”); random access memory (“RAM”); magnetic disk storagemedia; optical storage media; flash memory devices; etc.

A Wi-Fi Communication System

One embodiment of a Wi-Fi wireless communication system is described. Inthe Wi-Fi communication system, communications occur at 2.4 GHz or 5.8GHz. Note that in alternative embodiments, communications in thewireless communication system occur at other radio frequencies.

In one embodiment, the Wi-Fi communication system is used as part of anintelligent sensor network having one or more sensor tags. The sensortags harvest and store RF energy (e.g., Wi-Fi, cellular, etc.), collectand process sensor data, and communicate with other devices (e.g., Wi-Fidevices) using a communication standard (e.g., Wi-Fi, Zigbee, Bluetooth,Bluetooth Low Energy), or even proprietary interfaces. In one embodimentin which the sensor tags communicate via Wi-Fi, the sensor tagcommunicates with another Wi-Fi device by sending standard UDP packets.In another embodiment in which the sensor tags ultimately communicatevia Wi-Fi, the sensor tag communicates using backscatter communication(a backscatter to Wi-Fi bridge is used in this case). Wi-Fi devicesperform energy harvesting and storage and utilize a wake-up and commandprotocol to wake-up, perform sensing and communicate using standardcommunication (e.g., Wi-Fi, Bluetooth Low-Energy, etc.). In oneembodiment, the sensor tag also performs operations (e.g., commands)that may or may not be based on the sensed data.

In one embodiment, the intelligent sensor network includes low-powerWi-Fi sensor tags that use a low-power Wi-Fi state machine to controltheir operation. FIG. 1 illustrates one embodiment of a sensor tag 100that communicates over Wi-Fi. Referring to FIG. 1, antenna 101 iscoupled to switch 103. The impedance matching circuits 102 a and 102 bon the outputs of the switch may comprise a passive network ofcomponents that improves energy transfer from a source impedance to aload impedance. In one embodiment, impedance matching circuits 102 a and102 b are LC circuits. Switch 103 has a terminal coupled to RF radio 105via impedance matching circuit 102 a. In one embodiment, RF radio 105 isan 802.11 Wi-Fi radio, such as a radio from GainSpan of San Jose, Calif.RF radio 105 is coupled to microprocessor 106. Another terminal ofswitch 103 is also coupled to energy harvesting and storage circuitry104 via impedance matching circuit 102 b. Energy harvesting and storagecircuitry 104 is used to provide power to microprocessor 106 and thesensors 111. In one embodiment, energy harvesting and storage circuitry104 includes an energy harvesting unit and a storage unit. Theharvesting circuitry may include a diode based rectifier for convertingincoming RF energy to a DC voltage. In some embodiments, the diode basedrectifier may include Schottky diodes such as those manufactured byAvago Technologies Inc. The harvesting circuits may also include energymanagement functions based on discrete implementations known to thosefamiliar with the state of the art, or they could use parts such as theMaxim 17710 or the Linear Technology LTC3108. The storage unit can be acapacitor, super-capacitor, or any type of rechargeable batterytechnology such as, for example, a Thinergy MEC201. The tag materialitself can be a standard printed circuit board, or a flexible tagprinted on film such as modern standard RFID tags.

Tag 100 spends most of its time asleep. In order to sense, orcommunicate, it needs to know when to wake up. This wake up can be atpre-determined intervals pre-configured into microprocessor 106. In oneembodiment, tag 100 is awakened prior to its self-wakeup time. In oneembodiment, this is caused by use of an external interrupt frominterrupt device 110. Interrupt input device 110 and sensors 111 arealso coupled to microprocessor 106. In one embodiment, interrupt inputdevice 110 provides an interrupt to microprocessor 106 to wake-up tag100. The interrupt may be in response to an action. In one embodiment,interrupt input device 110 is a button which provides an interrupt tomicroprocessor 106 in response to the button being depressed. In anotherembodiment, interrupt input device 110 comprises a motion sensing devicethat generates an interrupt to microprocessor 106 in response to motionoccurring in proximity to the tag 100. In yet another embodiment,interrupt input device 110 comprises a microphone device that providesan interrupt to microprocessor 106 in response to capturing sound thatis audible in proximity to tag 100. In still another embodiment,interrupt input device 110 generates an interrupt periodically or atpre-defined response time-slots to wake-up microprocessor 106 or aself-timer interrupt can be running autonomously on microprocessor 106.Interrupt input device 110 can also be any other type ofinterrupt-generating device, known to those familiar with the state ofthe art, other than the example embodiments shown here.

Sensors 111 include one or more sensors that sense data and providesensed data to microprocessor 106. In one embodiment, sensors 111comprise one or more temperature, pressure, humidity, gas composition,image, and position sensors. In one embodiment, in response to one ofsensors 111 sensing data, the sensor generates an interrupt tomicroprocessor 106 to wake-up microprocessor 106 so that the sensed datacan be stored on the tag (in RAM or ROM internal or external tomicroprocessor 106), so that it can be uploaded to the network (viawireless communication with another RF device that is proximate to it)at a later time. In one embodiment, these sensors 111 are periodicallypolled by microprocessor 106 (at a pre-configured polling rate).However, this can be costly from an energy usage perspective. In oneembodiment, sensors 111 interrupt microprocessor 106 only when theirsense outputs change significantly enough to desire microprocessor 106to wake up and capture the new condition prior to going back to sleep.Finally, a sensed situation might be significant enough (such as analarm alert) that one of sensors 111 wakes microprocessor 106 up for acommunications event, in addition to a storage event.

Energy harvesting and storage circuitry 104 receives energy via antenna101 through switch 103 and impedance matching circuit 102 b duringenergy harvesting. Energy harvesting may occur when microprocessor 106is asleep, and need not occur when microprocessor 106 is communicating.The energy harvested is stored in a energy storage device such as, forexample, a capacitor or battery. In one embodiment, when the tag isperforming computations or other functions, apart from energyharvesting, harvesting energy and storage circuitry 104 provides powerto microprocessor 106.

FIG. 2 is a tag state diagram for one embodiment of the tag of thepresent invention and represents the various states the tag may be induring its use. In one embodiment, the tag state diagram depicts thestate of tag 100 of FIG. 1.

Referring to FIG. 2, while in the harvest-and-sleep state 201, the tagperforms energy harvesting and storage and is in the sleep state. Thus,the tag does not continually monitor the RF (e.g., Wi-Fi) traffic. Thetag remains in state 201 waiting for an interrupt to occur. Theinterrupt may be an action, event, or the result of data being sensed byone of the tag's sensors.

In response to a sensor interrupt from one or more sensors, the tagtransitions into the wake-and-sense-and-store state 205. In oneembodiment, in the wake-and-sense-and-store state 205, the tag wakes up,senses the state from one or more of the tag sensors, such as sensors111 of FIG. 1, and stores the data. Afterwards, the tag transitions toharvest-and-sleep state 201.

In response to an action interrupt (a communication interrupt) such asfrom an interrupt input device, from a self-timer, or from an alertsensor condition, the tag transitions into the wake-and-sense state 202.In one embodiment, in the wake-and-sense state 202, the tag wakes up andsenses the state from one or more of the tag sensors, such as sensors111 of FIG. 1.

After sensing the state, the tag transitions into the upload state 203in which the tag uploads (transmits) data to the network. In oneembodiment, the data that is being uploaded is sensor results or othertag response data, which may include, but are not restricted to: tagunique identifiers, date/time, tag functional status, configuration andfirmware update download status, and tag configuration parameters. Thetag may also upload the result of a function computed on the sensorresults or other tag response data. For example, a tag that sensestemperature may store in a RAM or ROM of the tag a history of recenttemperature readings and if a trend is detected that indicates ananomaly, such as a rise (or drop) over time, or an unusual pattern ofrises and falls, an alert message can be sent. A tag that captures andprocesses images can send an alert when an unexpected object is seen,such as, for example, an out-of-place item on an assembly line, anintruder in a secure facility, or that an elderly person has fallen downand cannot get up. The tag may also monitor and store a record of itsinternal operation. This is useful for monitoring the performance of thetag itself as well as the energy and communication environment thatsupports the tag. Examples of internal operational information includethe number of times the tag's energy storage has been fully charged, thenumber of times the tag has started harvesting energy, the amount ofcharge in the energy storage when harvesting started, the number ofinterrupts received, the number of uploads performed, the number ofacknowledgement messages transmitted, etc. If a clock is available, thetimes when these events occurred could also be recorded. The informationabout the internal operation of the tag can be uploaded upon receipt ofthe appropriate command. In one embodiment, the uploading occurs as partof a message packet (e.g., UDP packet).

After uploading to the network has been completed, the tag transitionsto the receive-and-acknowledge state 204. While in thereceive-and-acknowledge state 204, the tag waits for an acknowledgement.If the acknowledgement is received, the tag transitions back into theharvest-and-sleep state 201 where the tag waits in the sleep state foran interrupt. In one embodiment, if an acknowledgement is not receivedwithin a predetermined period of time, and a timeout occurs, the tagtransitions from the receive-and-acknowledge state 204 to theharvest-and-sleep state 201, but logs the error condition that can bereported on the next network upload. Note that in one embodiment, thetag may transmit one or more copies of the same data to ensure deliverywithout waiting for an acknowledge packet.

In one embodiment, while in the receive-and-acknowledge state 204, thetag may receive an optional command packet prior to receiving theacknowledgment packet. In one embodiment, one optional command is acommand that provides configuration information to the tag, which setsor changes the tag's configuration upon execution. A non-exhaustive listof tag configuration examples includes: a bitmask of enabled/disabledtag sensors; a vector of sampling rates per sensor; the rate oftag-to-reader communications; a destination URL (or other resourcelocator) for tag data (the reader routes the data packet to that URL ontag's behalf). In another embodiment, one optional command is a commandwhich, when executed, disables all or a portion of the tag. One exampleof such a command would be a “kill” command, which permanently rendersthe tag non-operational. In another embodiment, one optional command isan upgrade message to the tag that initiates a firmware update. Thefirmware update procedure may involve several commands sent to the tag,with each command carrying a portion of the firmware. In one embodiment,the tag acknowledges each properly decoded command, and continues towait for the next command from a reader until the tag receives the finalacknowledgement packet (which can be but is not limited to a packet withthe continuation flag cleared).

In its simplest form, microcontroller 106 has a single code-space andconfiguration space, defining the operation of the microcontroller everytime it wakes up. However, especially in the case of a firmware update,if the update is not properly loaded, the firmware may be incomplete,which may render the microcontroller inoperable, and the tag 100non-functional. A typical method to overcome this problem is formicrocontroller 106 to have two code spaces—one active code space, andone download/update code space. Only when the download/update code spaceis validated (a CRC checksum is sufficient) does it become the primaryoperating firmware. Microcontroller 106 can simply switch active withdownloaded code spaces by changing program pointers, or by actuallycopying all the code. Typically, there is a section of “protected” codethat can never be changed—this especially includes primary boot-upfunctionality, as well as fundamental communication and debugging tools.All of these features can be supported by microcontroller 106 on tag100.

In addition, there may be one, or multiple, configuration spaces inmicrocontroller 106 on tag 101. The microcontroller can be configured toalways use the same configuration option, or it can move throughconfiguration options in a pre-determined manner every time it wakes up.Configuration options may specify different sensing frequencies, sensortypes, or different types of sensor data storage depending on theapplication. Configuration options may also specify different hostcommunication protocols. This way, when the tag uses a differentconfiguration option every time it wakes up, the tag is automaticallyreconfiguring its own operational state.

Thus, while in the receive-and-acknowledge state 204, the tag mayreceive a complete firmware update of new active code, and/or newconfiguration options. Alternatively, especially because the tag may nothave sufficient energy to receive this amount of download data, only asubset of this data may be received during one wakeup event. In oneembodiment, only a subset of this data is required (such as updating asingle configuration option). In any case, the packet sent by the tag inwake-and-sense state 202 can include the status of firmware andconfiguration updates. In this way, when the tag reachesreceive-and-acknowledge state 204, the host can continue thefirmware/configuration update from where it left off. After multiplewake-up events, eventually, the complete firmware update will bereceived by tag 100, and the new active code, and/or configurationoptions, can be implemented by the tag and microcontroller 106. This ishow a tag can reconfigure itself.

In the case where the tag is waiting for an optional command while inthe receive-and-acknowledge state 204, and thereafter completes thecommand, the tag transitions to the harvest-and-sleep state 201.

One Embodiment of a Backscatter Communication System

In one embodiment, the tag is part of a sensor network that usesbackscatter for communication. In backscatter communication, acontinuous-wave signal from a reader is modulated at the tag byreflecting (or not) data back to the reader. In the case of the presentinvention, the reader is another wireless device, such as a Wi-Fi accesspoint designed to interrogate a backscatter tag.

FIG. 3 illustrates one embodiment of a tag that communicates usingbackscatter. In one embodiment, the tag communicates using backscatterin the 2.4 GHz frequency band (but backscatter communications are notlimited to this frequency).

Referring to FIG. 3, tag 300 includes antenna 301 that receives Wi-Fi(or other RF) communications and is coupled to passive RF patterndetector 321 via impedance matching circuit 302 a and switch 303. In oneembodiment, the impedance matching circuit 302 a comprises an LCcircuit.

Backscatter communications, by its nature, requires energy from thereader (e.g., an access point) in order for the tag to communicate.Therefore, the tag needs to “listen” for the incoming RF energy, to beable to backscatter the energy to communicate. As a result, unlike theWi-Fi tag described above (which is only harvesting when asleep), thebackscatter tag actively “listens” for energy, even when it isharvesting. This listening can consume so much power that it is possiblethere won't be any power left to harvest. Thus, passive RF patterndetector 321 needs to be extremely low power (O-power ideal) so that itcan detect when a reader wants to communicate with the tag, but stillpermit sufficient energy to be harvested. Passive RF pattern detector321 monitors RF traffic and performs pattern matching on the receivedcommunications. The pattern is provided by another RF device near tag300. In one embodiment, this RF device is an access point (AP) near tag300 that provides energy for tag 300. If a matching pattern is detected,passive RF pattern detector 321 generates an interrupt tomicroprocessor/sensor unit 323. In one embodiment, the pattern is asequence that is unique to the tag. By using the unique sequence,passive RF pattern detector 321 is able to determine when communication(or RF energy) is directed specifically to this tag 300 (and other tagsdisregard this communication). In one embodiment, passive RF patterndetector 321 comprises a 150 nA microprocessor supervisor IC and a 200nA comparator from Austria Microsystems. The first acts as a delayelement for the incoming energy, while the second acts as a logic ANDgate, so that when a delayed pulse matches up with a second energypulse, the pattern is thus detected. In another embodiment, passive RFpattern detector 321 comprises two ultra-low-power operationalamplifiers (such as the OP281 from Analog Devices), configured as awindow comparator, designed so that a rectified voltage received fromthe antenna is confirmed to be between low and high voltage settings,thereby indicating a match between the incoming wave energy and theappropriate duty cycle, so that the incoming energy wave can bedetected. The passive RF pattern detectors described above aretime-based detectors. In addition, an embodiment of a frequency-based RFpattern detector will be described in more detail later in thisdescription.

In response to an interrupt from passive RF pattern detector 321,microcontroller 323 wakes up and performs one or more operations asdescribed herein. In one embodiment, microcontroller 323 may beimplemented with a microprocessor. In one embodiment, microcontroller323 wakes up in response to sensor input, pattern detector, or pre-settimer.

Switch 303 is coupled to demodulator 322 via impedance matching circuit302 b and energy harvesting and storage unit 304 via impedance matchingcircuit 302 c. During operation, switch 303 also receives RFcommunications received by antenna 301. While energy harvesting, the RFenergy received through switch 303 from antenna 301 is stored in energyharvesting and storage circuitry 304. When no longer harvesting energy(for example the energy storage may be full, or a wake-up event has beendetected), and performing and operating in the awake state, energyharvesting and storage circuitry 304 provides power to microprocessorand sensors 323.

When no longer harvesting energy (as described above), communicationsreceived from antenna 301 received through switch 303 are alsodemodulated by demodulator 322 and the demodulated communications areprovided to microcontroller 323.

In one embodiment, tag 300 includes a backscatter modulator 324 that iscoupled to microcontroller 323 and also coupled to antenna 301.Microcontroller 323 provides backscatter control to backscattermodulator 324 to provide a different modulation back to antenna 301 tocommunicate with another Wi-Fi device (via a backscatter communicationbridge) in its proximity. In the case of most basic modulation, the ASK(amplitude shift keying), modulator 324 can be as simple as a transistorconnecting a switchable impedance (which may include one of a shortcircuit, a resistance, a capacitive reactance, or an inductivereactance) from the antenna 301 to ground, or leaving it disconnected inresponse to a command from microcontroller 323. Modulator 324 may bemore sophisticated if a different modulation mode is required, like PSKor QAM. When modulator 324 connects the antenna to ground directly (incase of ASK modulation) or via circuit(s) with reactive components(e.g., a RC-circuit (PSK or QAM modulation), a part of incident RFenergy received by antenna 301 is reflected back to the reader and canbe recovered and detected by the receiver.

FIG. 4 is one embodiment of a backscatter tag state diagram. In oneembodiment, the tag state diagram illustrated in FIG. 4 represents thestates of tag 300 of FIG. 3.

Referring to FIG. 4, the tag is initially in a harvest-and-sleep-and-RFpattern detection state 401. During this time, the tag loops and waitsuntil an interrupt is detected.

In response to a sensor interrupt from one or more sensors on the tag,the tag transitions into the wake-and-sense-and-store state 405. In oneembodiment, in the wake-and-sense-and-store state 405, the tag wakes up,senses the state from one or more of the tag sensors, and stores thedata. Afterwards, the tag transitions to harvest-and-sleep-RF patterndetect state 401 and goes back to sleep.

While in the harvest-and-sleep-RF pattern detect state 401, in responseto a different type of interrupt being detected (not a wake/sense/storeinterrupt, but a communicate interrupt, which may be but is not limitedto a pattern detection interrupt), the tag transitions to thewake-and-sense state 402. While in the wake-and-sense state 402, the tagwakes up and performs a sensing action.

After completing the sensing operation, the tag transitions tobackscatter transmit state 403 in which the tag transmits a backscatterpattern to communicate via backscatter. The sensor data is transmittedfrom the tag to the reader in this state. Additional data and tag statusmay be communicated at this time as well, as detailed in the standardcommunications (i.e. “Wi-Fi) embodiment described earlier. Afterperforming the backscatter transmission, the tag transitions toreceive-and-acknowledge state 404.

While in the receive-and-acknowledge state 404, the tag waits for anacknowledgement. If the acknowledgement is received, the tag transitionsback into the harvest-and-sleep state 401 where the tag waits in thesleep state for an interrupt. In one embodiment, if an acknowledgementis not received within a predetermined period of time, and a timeoutoccurs, the tag transitions from the receive-and-acknowledge state 404to the harvest-and-sleep state 401, and an error can be logged andreported during the next communication sequence. Alternatively, one ormore messages may be backscattered, without waiting for anacknowledgement, because this may be less-expensive from an energyconsumption point of view.

In one embodiment, while in the receive-and-acknowledge state 404, thetag also waits for an optional command to complete. In one embodiment,one optional command is a command that provides configurationinformation to the tag, which sets or changes the tags configurationupon execution. In another embodiment, one optional command is a commandwhich, when executed, disables all or a portion of the tag. In anotherembodiment, the software (or firmware) on the tag may be updated viathis command. In the case where the tag is waiting for an optionalcommand while in the receive-and-acknowledge state 404, and thereaftercompletes the command, the tag transitions to the harvest-and-sleepstate 401.

In the earlier “Wi-Fi” communication embodiment description, there is adetailed explanation of how a tag may re-configure its own operationalparameters, as well as re-configuring its own software. In thisembodiment of a backscatter communication system, the tag may supportthe same functionality. The primary difference is that the communicationis carried out via a backscatter communication network, as opposed to astandard protocol (i.e., Wi-Fi) communication network.

FIGS. 5A-5C illustrate the backscatter tag of FIG. 3 performingbackscatter communication with an access point (AP) in a sensor network.Referring to FIG. 5A, AP 501 transmits a ‘wake-up’ sequence to tag 502.The passive RF pattern detector of tag 502 detects the sequence andwakes up/interrupts the tag's microcontroller. Energy is being harvestedby tag 502, as is normal.

Referring to FIG. 5B, the microcontroller of tag 502 wakes on thepattern detection interrupt, switches to communications mode (no longerharvesting energy), draws power from energy storage (e.g., a battery) intag 502, confirms the incoming wakeup sequence is directed to itself,and communicates a response via backscatter to AP 501. In oneembodiment, the response is in the form of a hybrid backscatter-Internetpacket that is easily converted to a format compatible with Internetcommunication. This allows the destination IP address for the packet tobe set on the tag rather than the reader, thereby overcoming asignificant limitation of many backscatter communication systems. Thedestination IP address (and other communication parameters such as portnumbers) can be set statically at the time the tag is installed or itcan be computed dynamically at run time. In one embodiment, the datacomputed by a tag is packaged in a User Datagram Protocol (UDP)-readyformat. This is often used in networked applications that transmitlimited amounts of information, such as sensor readings, that need toeliminate any requirement for prior communication to set up specialtransmission channels or data paths. This reduces complexity and powerrequirements and is well suited for the backscatter model ofcommunication in which the physical channel can be unreliable andintermittent. A more detailed description of the hybrid backscatter-UDPpacket follows soon.

Referring to FIG. 5C, AP 501 receives the response from tag 502 andsends an optional acknowledgement. In one embodiment, AP 501 also sendscommunications or configuration information to tag 502.

At this point, communications between AP 501 and tag 502 are complete,and tag 502 returns to its initial state (i.e., FIG. 5A) and AP 501returns to perform its other AP functions. Before going to deep sleep,tag 502 configures itself for harvesting energy again, and passivelylooks for the RF pattern to wake up once again.

FIG. 6 shows a hybrid backscatter-UDP packet. The information in astandard UDP packet (transport layer packet) includes a payloadcontaining application data from a sensor reading. This is encapsulatedin a IP packet (link-layer packet), and in turn encapsulated in abackscatter frame (a MAC-layer frame). This is conceptually similar toIEEE802.3 (Ethernet) or IEEE 802.11 (radio frame) encapsulation.Referring to FIG. 6, a backscatter frame 600 can include a preamble 601,running frame ID 602, destination MAC (MAC of the Reader) 603, sourceMAC (MAC of tag) 604, length 605 of payload 606, and a frame sequencecheck 607 (may be a checksum). The UDP packet 606 is an encapsulated IPpacket. Any standard IP packet format can be used such as IPv4, IPv6, or6LoWPAN. The IP packet 606 may include headers 609, source address oftag 610, destination address of UDP packet 611, protocol number 612(UDP: 17, or 0x11), length 613 of payload 614, and the payload of theactual UDP packet 615. The tag IP address 610 can be an auto-assignedEUI-64-based link-local address, a self-assigned IPv4, a DHCP-assignedone, etc. The UDP packet 615 can include UDP headers 616, optionalsource port 617, destination port 618, length 619 of UDP payload 621,and checksum 620. The payload 621 is the actual tag data. In oneembodiment, the backscatter frame 600 is further optimized by omittingor compressing known elements of the frame in a manner similar to the6LoWPAN standard. With addresses, ports, headers, and other routinginformation, AP 501 can efficiently route the packet (and it provides amuch easier integration to the cloud or other network arrangement). Thatis, the standards-compliant encapsulation of data within a UDP packetand an IP packet simplifies the computation performed by AP 501. Itmerely copies that data into a standard frame and transmits it as itwould any other link layer frame. In this way the backscatter-UDP packetis auto-routed onto the network by AP 501.

In an alternative version of the packet format, part or all of thestandard UDP and IP packet formats could be disregarded and a customformat adopted. That format would need to be understood by AP 501 sinceit would need to convert it to a UDP packet. In one implementation, thetag provides a destination IP address and data and AP 501 computes therest. Alternatively, the tag provides data from which an IP addresscould be computed such as, for example, the ID number of a node in anetwork. Any packet format that contains a large amount of data, such asthe hybrid UDP-backscatter packet or any alternative custom format, willhave an issue with reliability of reception. As the length of the packetincreases, the probability that it will be correctly received decreases.This sometimes is caused by the intermittent nature of some backscattercommunication. Because of this characteristic, the communication betweenthe tag 502 and AP 501 may be performed with a streaming protocol thatsplits the packet into smaller chunks that are reassembled by AP 501before transmission on the network. For more information, see Buettner,et al., 2008, “Revisiting Smart Dust with RFID Sensor Networks,” HotNetsconference.

In one embodiment, the RF energy harvesting happens in the 2.4 GHz ISMband, while backscatter communications occur in the 5.8 GHz ISM band.FIG. 11 illustrates a wireless communication system that uses thisapproach. This approach has several benefits, namely:

-   -   2.4 GHz band is a more energy-abundant RF band, since it is most        popular with the current generation of Wi-Fi and other RF        devices;    -   5.8 GHz band is more suitable from communication perspective, as        it is less occupied, and also allows design of more        sophisticated antennas with roughly 50% reduction in linear size        of antenna structure; and    -   separation of RF energy harvesting and backscatter paths leads        to reduced complexity of the antenna and matching circuits.

Referring to FIG. 11, multiple 5.8 GHz antennas can be added in thespace of one 2.4 GHz antenna, to the access point, to the tag, or both.These antennas can take on the function of diversity, beamforming,and/or separate TX/RX depending on the needs. Although only a single 5.8GHz antenna is shown, alternative embodiments may have more than one.

Also, a separate passive RF pattern detector can be placed on the tag inthe 2.4 GHz signal path, to complement or to replace the one in the 5.8GHz path in FIG. 11.

An Alternative Frequency-Based RF Pattern Detector (Wakeup Method) forSensor Tags

In one embodiment, the passive wakeup pattern detector of the Wi-Fi orbackscatter tag comprises a frequency-based wakeup detector device. Inone embodiment, a sensor tag implements this frequency-based “passive RFpattern detector”, as a combination of two detectors: a “thresholddetector” block and a “pattern detector” (FIG. 10). The thresholddetector block constantly looks at the RF power in the sub-band ofinterest comprised of a set of subcarrier frequencies {f_(i)}, andgenerates a pattern detector wake-up interrupt if this power exceeds apower threshold A_(threshold). This threshold is set at a safe marginabove the larger of the pattern detector apparatus sensitivity value,and the typical ambient RF noise level. The pattern detector wake-upinterrupt wakes up the microprocessor.

In one embodiment, the pattern detector is comprised of a set ofparallel narrow band-pass filters, tuned to a set of subcarrierfrequencies {f_(i)}, with threshold detectors attached to each filter,and a Boolean function implementing a passive detection of a frequencypattern (i.e., FIG. 10):

-   -   listen on the set of subcarrier frequencies {f_(i)}    -   construct a logical Boolean vector {b_(i)} for a set of Wi-Fi        subcarriers i, where

$b_{i} = \left\{ \begin{matrix}{0,} & {{A\left( f_{i} \right)} < A_{{threshold}\mspace{14mu} i}} \\{1,} & {{A\left( f_{i} \right)} > A_{{threshold}\mspace{14mu} i}}\end{matrix} \right.$

-   -   use some Boolean function of F({b_(i)})==1 for a specified time        T_(wakeup) as a wakeup condition for the tag core electronics        Note that the power threshold A_(threshold) in the threshold        detector is loosely related to the set of thresholds in the        pattern detector, e.g. it could be given by a formula

$A_{threshold} = {\sum\limits_{(i)}\; A_{{threshold}\mspace{14mu} i}}$

In one embodiment, A_(threshold i) is variable. Also, the set offrequencies {f_(i)} does not have to be limited to one Wi-Fi channel,especially if it is desirable to respond to readers on different Wi-Fichannels.

FIG. 10 illustrates one embodiment of threshold and pattern detectors.The threshold and pattern detectors can be implemented in hardware, withlow-power envelope detectors, comparators, and Boolean logic elements,in software running on a low-power microprocessor, with multiple taps ofFast Fourier Transform (FFT) used as outputs of subcarrier frequencyfilters, or as combination of hardware and software, i.e. ahardware-only threshold detector, and a software-only pattern detector.In one embodiment, in the threshold detector block, the received RF isfirst passed though a bandpass filter F, where the transparency window Ftightly bounds the set of pattern detector frequencies {f_(i)}. Thefiltered signal is next fed into the envelope detector, and theresulting low-frequency RF envelope is fed to a comparator withreference level of A_(threshold). When the RF envelope value exceeds thethreshold value of A threshold, the wakeup interrupt is generated thatactivates the pattern detector. In one embodiment, the pattern detectoris comprised of a n-way RF splitter, feeding the RF signals to nparallel detector branches, each branch comprised of a narrow bandpassfilter tuned to a subcarrier frequency f_(i), feeding the filteredsignal into an envelope detector, and next into a comparator with athreshold of A_(threshold i). Boolean outputs of those comparators arefed into a Boolean logic block synthesizing the above-mentioned logicalfunction F. The general wake-up interrupt is generated when the logicalconditions encoded by the function F are finally met.

In one embodiment, a single frequency f_(i), or a set of subcarriers, isused on one channel only. This approach may have certain advantages fromthe overall interrogator/tag system behavior.

The tag may also respond to a set of symbol combinations, e.g. dependingon its state: use a “priority” symbol to send a mission-criticalresponse earlier, respond with different measurement communicated back,or respond with an action like blink an LED.

In one embodiment, the Wi-Fi reader periodically runs a sweep ofsupported wake-up symbols {f_(i)}_(k), k=1 . . . N, and runs the sweepuntil one of following conditions is met: a tag responds; a number ofsymbols is tried unsuccessfully; or the sweep set is exhausted. Onefeature of this method is that the “tag environment” of the reader canbe continually sampled and updated for new tags that may have movedinto, or out of, the read range of the access point (reader).

The sweep algorithm may pick up symbols from the set in a specific way,e.g., pick up symbols randomly, with certain weights (probabilities)assigned to symbols; goes through the symbol set sequentially; orcombines the two, going through a set of preferred, or priority, symbolsfirst, and then reverts to random draw for the rest of symbols/sweepduration.

Symbol preference may be assigned statically, dynamically, or both, toreflect: relative tag importance or acceptable communication latency(mission-critical tags); or known tag presence (keeps a dynamic table ofresponding tags count per symbol, with some expiration mechanism).

Wi-Fi Access Point Architecture

If Wi-Fi tags, such as those described in FIGS. 1 and 2, are used, thereare no required changes to the access points through which theycommunicate to a network (e.g., the Internet). However, if backscattercommunication is used, as described in FIGS. 3 and 4, access pointchanges will be required to facilitate the communications between abackscatter tag and the network (internet). There are a number ofembodiments of access points that may be used to enable a tagcommunicating via backscatter to be connected to a network. These areshown in FIGS. 7-9. Referring to FIG. 7, access point (AP) 700 uses awired connection (e.g., a USB connection or a Power-Over-Ethernetconnection) between itself and a backscatter Wi-Fi USB adapter 701. AP700 includes a CPU 711 coupled to an 802.11 radio 710. Radio 710 iscoupled to antenna 712. CPU 711 interfaces with backscatter Wi-Fi USBadapter 701 via USB interface 720 (optionally a Power-Over-Ethernet orequivalent interface). USB interface 720 is coupled to backscatter radio721, which is coupled to an optional backscatter modulator 722 and ademodulator 723. Both optional backscatter modulator 722 and ademodulator 723 are coupled to antenna 724. Optionally, the USBconnection can be a power-over-ethernet, or some other wired connection.Modulator 722 is used to transmit the energy and communication data fromthe access point (reader) to the tag. The energy can be continuous waveenergy, or it can be modulated to contain tag wake-up codes (which wakeup passive RF pattern detector 321). Modulator 722 is also used totransmit acknowledge or command instructions 404. Demodulator 723 isused to receive the backscattered signal back from the tag, anddemodulate the information into a data stream. This data stream containsthe sensor data captured by, stored in, and transmitted (backscattered)by the tag. In this way, backscatter adapter 701 acts as a bridgebetween backscatter communications with a tag, and the network.Alternatively, backscatter adapter 701 may optionally have two separateantennas, one to support transmitted energy and command/acknowledgedata, and one to receive the backscattered data back.

AP 700 also includes a wired connector for connecting AP 700, includingCPU 711, to a network (e.g., the Internet, the cloud, etc.).

In an alternative embodiment depicted in FIG. 8, the USB adapter 701from FIG. 7 is integrated directly into the access point. Referring toFIG. 8, AP 800 includes all the components, including CPU 711 coupled toboth 802.11 radio 710 and backscatter radio 721, which is coupled tooptional backscatter modulator 722 and demodulator 723. Optionalbackscatter modulator 722, a demodulator 723, and 802.11 radio 710 arecoupled to antenna 712. In one embodiment, AP 800 includes an additionalantenna, antenna 713. The number of antennas can vary with the needs andfunctions of Wi-Fi communication, diversity, and backscattercommunication. The operation of backscatter radio 721, the optionalmodulator 722, and demodulator 723, are identical in FIG. 8 to theiroperation in FIG. 7, but embedded within the access point as opposed toa separate device outside it.

AP 800 also includes a wired connector for connecting AP 700, includingCPU 711, to a network (e.g., the Internet, the cloud, etc.).

FIG. 9 illustrates an ASIC integration of backscatter communication intoan AP. Referring to FIG. 9, AP 900 includes a combined 802.11radio+backscatter radio chip 901 that is coupled to CPU 711 and antenna712. AP 900 also includes a wired connector for connecting AP 700,including CPU 711, to a network (e.g., the Internet, the cloud, etc.).In one embodiment, the functionality of radio 721, modulator 722, anddemodulator 723 are all performed by circuits designed into combinedradio 901. Radio 901 is advantageous because of tighter integration ofdesign, reducing size, cost, and power.

The process of transmitting information via backscatter communication(modulation) is well-known to those familiar with the state of the art,particularly in the field of RFID communication. The same is true forreceiving information via backscatter communication (de-modulation).However, the demodulation process is different whether it's coherent ornon-coherent.

FIGS. 7, 8, 9 are all constructed so that they support coherentde-modulation. The backscatter transmitter and receiver are co-locatedin the same device. The carrier frequency used to generate thetransmitted data on the backscatter transmitter can be used as areference frequency in the backscatter receiver, to help demodulate thereceived data. In this way, the signal to noise ratio is improvedsignificantly, because it is possible to easily synchronize the receiveddata with the transmitted data. These conditions support longercommunication distances, as well as higher data bit rates (greaterbandwidth).

However, in some cases, the backscatter receiver will be remote from thebackscatter transmitter. For example, a transmitter may be a mobiledevice (a custom-designed apparatus or a mobile phone), and the receivermay be a modified access point in a fixed location. This condition iscalled non-coherent demodulation. The backscatter receiver is locatedseparately from the transmitter, and therefore the receiver does nothave access to the carrier frequency of the transmitted data. In thiscase, demodulation is still possible (the process is known to thosefamiliar with the state of the art), but transmission distances andbandwidths may be smaller. In this case, it is also possible topre-negotiate the carrier frequency (or include the frequency in thetransmitted/received packet definition) to facilitate bettersynchronization at the demodulator.

Examples herein do not imply a preference of one modulation scheme overanother (coherent vs. non-coherent), and this system design supports allconfigurations.

Finally, because of the double-path-length signal losses inherent inbackscatter communication, it may be beneficial to construct bridgenodes physically located in between an Access Point (transmittingdevice) and a tag. These bridge nodes, being positioned closer to thetag, may be better able to receive the backscatter communication. Thebridge nodes may then pass on the data to the access point via astandard wired or wireless communication interface (ethernet, Wi-Fi,Bluetooth, Zigbee, etc.). In this case, the backscatter system can bedeployed with a minimum number of access points, coupled with bridgenodes reducing backscatter transmission distances, for maximum signaltransmission quality and bandwidth.

The means of providing security/privacy for backscatter communicationsystems are well-known to those familiar with the state of the art,particularly in the field of RFID communication. The communicationtechniques presented herein are compatible with standard securityprotocols.

Whereas many alterations and modifications of the present invention willno doubt become apparent to a person of ordinary skill in the art afterhaving read the foregoing description, it is to be understood that anyparticular embodiment shown and described by way of illustration is inno way intended to be considered limiting. Therefore, references todetails of various embodiments are not intended to limit the scope ofthe claims which in themselves recite only those features regarded asessential to the invention.

We claim:
 1. An RF tag for use in a sensor network, the tag comprising:a radio; an energy harvesting unit operable to convert incident RFenergy to direct current (DC); a storage unit operable to storerecovered DC power; one or more sensors for sensing and logging data; amicrocontroller coupled to the energy harvesting and storage units, theone or more sensors and the radio, the microcontroller operable to wakeup from a sleep state and cause the radio to communicate sensed datafrom at least one of the one or more sensors while powered by energypreviously harvested and stored by the energy harvesting and storageunit.
 2. The tag defined in claim 1 wherein the microcontroller isoperable to wait in a sleep state until receiving an interrupt that isin response to an RF command sequence, a sensed event, or a userstimulus.
 3. The tag defined in claim 2 wherein the RF command sequencecomprises a modulated pattern, a specific time domain energy signature,or a specific frequency domain energy signature.
 4. The tag defined inclaim 1 wherein the microcontroller causes the radio to communicatesensed data in response to the interrupt.
 5. The tag defined in claim 1further comprising: an antenna; and a switch coupled to interface theantenna with the radio and the RF energy harvesting and storage unit. 6.The tag defined in claim 1 wherein the microcontroller is operable towake up when receiving an interrupt, and in response, causes: data to besensed by the at least one sensor; uploading the sensed data to thenetwork; and waiting to receive an acknowledgement that the sensed datawas received.
 7. A Wi-Fi communication system comprising: an accesspoint coupled to provide access to a network; an RF tag to communicatewirelessly with the access point, the tag comprising a radio; an energyharvesting unit operable to convert incident RF energy to direct current(DC); a storage unit operable to store recovered DC power; one or moresensors for sensing and logging data; and a microcontroller coupled tothe energy harvesting and storage unit, the one or more sensors and theradio, the microcontroller operable to wake up from a sleep state andcause the radio to communicate with the access point via Wi-Fi to sendsensed data to the network from at least one of the one or more sensorswhile powered by energy previously harvested and stored by the energyharvesting and storage unit.
 8. The communication system defined inclaim 7 further comprising an interrupt input device coupled to providean interrupt to the microcontroller.
 9. The communication system definedin claim 8 wherein the interrupt input device is associated with asensor, a timer, or other interrupt-generating source.
 10. Thecommunication system defined in claim 8 wherein the microcontrollercauses the radio to communicate sensed data in response to theinterrupt.
 11. The communication system defined in claim 7 wherein theRF tag communicates using as a radio protocol a Wi-Fi standard interfaceso that it is compatible with existing Wi-Fi systems.
 12. Thecommunication system defined in claim 7 further comprising: an antenna;and a switch coupled to the antenna with the radio and the RF energyharvesting and storage unit.
 13. The communication system defined byclaim 12 further comprising an impedance matching circuit coupling theantenna and the switch.
 14. The communication system defined in claim 7wherein the one or more sensors includes one or more of a temperaturesensor, a pressure sensor, a humidity sensor, a gas composition sensor,an image sensor, and a position sensor.
 15. The communication systemdefined in claim 7 wherein the microcontroller waits in a sleep stateuntil receiving an interrupt, and in response, causes: data to be sensedby the at least one sensor; upload the sensed data to the network; andwait to receive an acknowledgement that the sensed data was received.16. The communication system defined in claim 7, wherein themicroprocessor logs an error code if timeout occurs while waiting forthe acknowledgement, the microprocessor causing the error code to bereported when the tag next wakes up.
 17. The communication systemdefined in claim 7 wherein the microcontroller transitions into a sleepstate in response to receiving the acknowledgement.
 18. Thecommunication system defined in claim 7 wherein the microcontrollerwakes up from a sleep state in response to at least one of a senseinterrupt, a timer interrupt, a wake-on-change, or being polled and inresponse to the sense interrupt causes the at least one sensor to senseand store data, the microcontroller returning to the sleep state withoutcommunicating the sensed data at that time.
 19. The communication systemdefined in claim 7 further comprising one or more other RF devices toprovide RF energy that is harvested by the harvesting and storage unitof the tag.
 20. An RF tag for use in a sensor network, the tagcomprising: a radio; an energy harvesting unit operable to convertincident RF energy to direct current (DC); a storage unit operable tostore recovered DC power; one or more sensors for sensing and loggingdata; and a microcontroller coupled to the energy harvesting and storageunits, the one or more sensors and the radio, the microcontrolleroperable to wake up from a sleep state and cause the radio tocommunicate sensed data from at least one of the one or more sensorswhile powered by energy previously harvested and stored by the energyharvesting and storage unit, and wherein the microcontroller, responsiveto an acknowledgement received via the radio, is operable tore-configure the RF tag based on command information in theacknowledgement.